Brands and Plate Quality. 281 be well squeezed in both directions when made into plate. Steel plates are much more reliable now than when first introduced, it being clearly recognised that a certain amount of strength must be sacrificed to ductility. They are not, therefore, considerably stronger than* iron, but much more homogeneous or even in structure, the particles being so thoroughly re-arranged when in the molten state. Iron plates, on the other hand, are very various in quality, even over one plate,1 because of the processes employed in obtaining fibre. A test strip, either for plate or bar, rarely gives an exact determination of the 'whole, while* the contrary holds with steel. Steel plates are termed c mild' because they have little more carbon than wrought iron plates ; they have some 30 per cent, more strength than the latter, with one and.a half to twice the elongation. The price of iron being also greater, it is not surprising that steel is the only material now used for plate work, excepting where continuous flame action (as in locomotive fire- boxes) renders Lowmoor iron or copper preferable. Copper, though more expensive, and losing its strength somewhat while hot, is an "exceedingly good conductor of heat, and deteriorates less under the action of flame, lasting therefore longer than iron, while being more efficient Steel, then, in a mild, homogeneous form, is the material now generally used for all plate and angle work. Both steel and iron are received from the rolling mill under the following forms: Plates --------- Angle Bais L Tee Bars T Channel Bars n H (Aitch) Bars H Flat and Square Bars Round Bars Brands, qualities, and sizes of plates.The qualities of iron have been mentioned at p. 76, * common' being used for bridges and girders, and the remainder for boiler work. Mild Steel occurs in four qualities, thus : 1. Ship and bridge quality. , 2. Ordinary boiler quality. 3. Soft boiler quality. 4. Superior quality (to resist flaime).